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A tone decoder is a device that can interpret the sound frequencies and their wavelengths on telephones and other digital tone devices. They have been in use since at least the 1960s. One of their first uses was by telephone operators, who helped clients to make long distance calls. They can also be utilized as a signal for local cable television advertisement, and as a device for espionage in tracking calls made from and to a telephone.
Tone frequencies on a telephone have low and high components and are unique in character. Since a telephone tone has two frequencies, it is known as a dual tone multi-frequency, or DTMF tone. There are 16 such tones that may be utilized with this technology, but, typically, a phone uses 12. The tone decoder works by reading low and high frequencies and pairing them to identify the tone.
In addition, every touch-tone telephone is equipped with a tone decoder. It allows calls to be generated with the buttons, which are pressed on the phone. As the button is pressed, the decoder takes information from the pressed button, sending the proper signal in tone form to complete the call.
There are several other uses for tone decoder technology. One practical use can be observed in a caller identification unit — the unit displays the number that has called by deciphering the tones that are sent from that number. Further, emergency personnel, such as firefighters, often use this technology to monitor scanner radios for the unique tones broadcast from places, such as firehouse, calling them to respond to an emergency.
Radio hobbyists, or anyone else who might be interested, can build a tone decoder with a few simple parts for very little expense. Sometimes, students make them as a science experiment or project. Once they are made, they have many practical uses. If attached to a telephone, with some additional equipment, they can be used to turn on and off lights, radios, and many other appliances from a remote location.
In espionage — sometimes to avoid detection, but to also provide useful information — a tone decoder is used instead of other intelligence-gathering devices. Tone decoders can detect a busy signal, as well as incoming or outgoing telephone numbers. This can give the person monitoring the line information, like frequently called numbers and how often a phone is used.